An Li2O—Al2O3—SiO2-based crystallized glass has been conventionally used as a material for a front window for a kerosene stove, a wood stove, or the like, a substrate for a high-tech product such as a substrate for a color filter or an image sensor, a setter for baking an electronic part, a tray for a microwave oven, a top plate for induction heating cooking, a window glass for a fire prevention door, or the like. For example, Patent Literatures 1 to 3 each disclose an Li2O—Al2O3—SiO2-based crystallized glass comprising an Li2O—Al2O3—SiO2-based crystal, such as a β-quartz solid solution (Li2O·Al2O3·nSiO2 (provided that 4>n≧2)) or a β-spodumene solid solution (Li2O·Al2O3·nSiO2 (provided that n≧4)), precipitated therein as a main crystal.
The Li2O—Al2O3—SiO2-based crystallized glass has a low thermal expansion coefficient and a high mechanical strength, and hence has an excellent thermal property. Further, appropriate adjustment of conditions of heat treatment in a crystallization step allows the kind of crystal precipitated in the Li2O—Al2O3—SiO2-based crystallized glass to be controlled, and hence a transparent crystallized glass (a β-quartz solid solution precipitates) can be easily manufactured.
When the crystallized glass of this kind is produced, a glass batch needs to be melted at high temperature exceeding 1400° C. Thus, used as a fining agent added in the glass batch is As2O3 or Sb2O3, which is capable of generating a fining gas in a large amount during the melting at high temperature. However, As2O3 and Sb2O3 are highly toxic and may pollute an environment, for example, in a production process of the glass or at the time of treating waste glass.
Thus, SnO2 and Cl have been proposed as substitute fining agents for As2O3 and Sb2O3 (see, for example, Patent Literatures 4 and 5). However, Cl is liable to erode a metal mold or a metal roll during glass formation, with the result that the surface quality of the glass maybe degraded. From the viewpoint of preventing such the problem, SnO2 is preferably used as a fining agent.
As described in Patent Literatures 4 and 5, SnO2 has a function of heightening coloring caused by Fe2O3, TiO2, or the like, and hence involves a problem in that a yellow tone of a transparent crystallized glass prevails, which is not preferred in terms of the outer appearance thereof. Thus, when SnO2 is used, it is preferred that the content of Fe2O3 contaminated as an impurity component be reduced and that the content of TiO2 in a glass batch be also reduced. However, TiO2 is a component of a crystal nucleus, and hence, when the content of TiO2 is reduced, an optimum firing temperature range becomes narrower, with the result that the generation amount of crystal nuclei is liable to be smaller. When crystallization progresses in the presence of a small amount of crystal nuclei, a large amount of coarse crystals are generated, causing a problem in that the crystallized glass is liable to become cloudy, thereby losing transparency.
As another method of suppressing the coloring of a transparent crystallized glass, there is known a method involving adding a colorant having a relationship of a complementary color with the coloring caused by Fe2O3, TiO2, or the like, thereby fading the coloring. It has been conventionally known that Nd2O3 is particularly effective for fading colors of the Li2O—Al2O3—SiO2-based crystallized glass (see, for example, Patent Literature 6). Thus, even when the yellow color tone of the Li2O—Al2O3—SiO2-based crystallized glass becomes prevailed by the addition of SnO2, such the yellow color can be faded by adding Nd2O3.
Note that it is possible to obtain a white opaque crystallized glass in which a β-spodumene solid solution is precipitated, by carrying out crystallization under a proper heat treatment condition in a production process of the Li2O—Al2O3—SiO2-based crystallized glass.